It's surprising to find that the 29 gallon is the smallest that's really easy. I started with a betta in a tiny tank for my kid, lost several fish, and now the new betta is ok in a 3 gallon with frequent water changes, but all the research I did in the mean time to learn how to keep a betta alive led me to the 29.
The simple guide to freshwater aquariums by Boruchowitz is awesome. Cheap on amazon and abe books, it's everything but fishless cycling.
It's really a shock how much there is to understand, to keep fish alive. When I was a kid my brother had a goldfish he win from a carnival live in a bowl for 7 years with very little care. I didn't realize that's extremely unusual.